The detection and treatment of ventricular arrhythmias by implantable devices is a proven technique for patients with serious heart conditions. Such devices contain an energy source, an electrode lead system in contact in the heart, a sensing system to detect the onset and type of arrhythmia, and a pulse generator for delivering pulses to treat the arrhythmia. Such devices may also include systems for detecting ventricular fibrillation, and for delivering defibrillation shocks.
One technique for detecting arrhythmias involves comparing the number of atrial P waves to the number of ventricular R waves occurring in an interval. This may be done by counting P waves and R waves as received by an endocardial lead system. If the two counts do not match, a determination of atrial or ventricular arrythmia can be made. However, if the P waves and R waves cannot be accurately discriminated from each other, the accuracy of the determination of arrythmia is impaired.
The reliable sensing of ventricular arrhythmia is critical, and is sometimes difficult because in many lead and sensing systems ventricular signals are combined or cross-coupled with atrial signals. Discrimination is further complicated by the fact that atrial and ventricular components will occur at different rates in the case of arrhythmias. This is a problem in devices which have only one lead in the ventricle, because discriminating atrial from ventricular arrhythmia is very difficult, and it is important to treat the patient appropriately. Examples of atrial arrhythmia include exercise (ST) or AF, AFL, SVT. Atrial arrhythmia is better left untreated by the ventricular defibrillator, but ventricular arrhythmia may be fatal and accordingly must be treated immediately.
Prior art techniques have generally relied on two strategies. One is to use two separate leads for the atrium and ventricle, with sensing electrodes positioned to maximize receiving the desired signals with minimum cross-coupling or unwanted contribution from the other. This technique can be successful, but requiring two separate leads is a disadvantage.
The other prior art technique is to use a single lead in the ventricle and to try to use electronic filtering to separate the atrial and ventricular signals received on this single lead. This has not been entirely successful in the prior art because the nature of the two signals, in terms of frequency and rate, makes them difficult to distinguish. In particular, tracking relatively small atrial P waves in the presence of larger far-field ventricular effects is very difficult in arrhythmia. Because of this, current devices with one ventricular lead cannot always discriminate atrial from ventricular signals, resulting in "inappropriate shocks".